The present invention pertains generally to magnetic tape drives, and more particularly to a variable speed data recording/recovery mechanism and method thereof.
The present invention is related to co-pending U.S. patent application entitled xe2x80x9cOverscan Helical Scan Head for Non-Tracking Tap Subsystems Reading at up to 1xc3x97 Speed and Method for Simulation of Samexe2x80x9d, invented by Blatchley et al., and having a Ser. No. of 09/176,013, filed concurrently herewith on Oct. 20, 1998, and copending U.S. patent application entitled xe2x80x9cFine Granularity Rewrite Method and Apparatus for Data Storage Devicexe2x80x9d, invented by Zaczek, and having a Ser. No. of 09/176,015, filed concurrently herewith on Oct. 20, 1998, and co-pending U.S. patent application entitled xe2x80x9cMulti-level Error Detection and Correction Technique for Data Storage Recording Devicexe2x80x9d, invented by McAuliffe et al., and having a Ser. No. of 09/176,015, filed concurrently herewith on Oct. 20, 1998, all of which are commonly owned and all of which are hereby incorporated by reference, and co-pending U.S. patent application entitled xe2x80x9cMethod And Apparatus For Logically Rejecting Previously Recorded Track Residue From Magnetic Mediaxe2x80x9d, invented by McAuliffe et al., and having a Ser. No. of 09/192,794, filed on Nov. 16, 1998, and co-pending U.S. patent application entitled xe2x80x9cMethod And System For Monitoring And Adjusting Tape Position Using Control Data Packetsxe2x80x9d, invented by McAuliffe et al., and having a Ser. No. of 09/193,030, filed on Nov. 16, 1998, and co-pending U.S. patent application entitled xe2x80x9cRogue Packet Detection And Correction Method For Data Storage Devicexe2x80x9d, invented by McAuliffe et al., and having a Ser. No. of 09/192,809, filed on Nov. 16, 1998, and co-pending U.S. patent application entitled xe2x80x9cA Method Of Reacquiring Clock Synchronization On A Non-Tracking Helical Scan Tape Devicexe2x80x9d, invented by Blatchley et al., and having a Ser. No. of 09/192,808, filed on Nov. 16, 1998.
Tape storage technology is routinely used for routine system back up and long-term data archiving. Various tape technologies such as DAT, DLT, and 8 mm, have been developed over recent years to meet industry needs for higher performance and increased data storage capacity. While they differ widely in format, capacity, and performance, conventional tape storage devices rely on some form of track-following architecture. Whether the underlying recording technology is based on linear serpentine or helical scan recording, track-following architecture tape drives attempt to read a track completely in a single pass using a single head assembly and operating at a single fixed tape speed. The drive mechanism and media tolerances are tightly controlled to maintain a very precise alignment between the path traced by the heads and the written tracks on a tape.
Track-following architecture tape drives save some common inherent limitations. To either write or read data, track-following tape technologies depend upon a constant head-to-tape speed for linear recording or constant track pitch for helical scan recording. Accordingly, the tape drive must either receive stream data at a constant transfer rate, or, when this does not occur, initiate a stop/backhitch/start sequence.
Due to normal speed variations found in network and workstation applications, the data transfer rate to or from the host rarely matches the tape drive""s fixed read or write speed. Whenever a mismatch occurs, the read or write operation is suspended and the tape is repositioned backwards to allow enough space to accelerate again to the forward operating speed. The time required for this repositioning cycle, known as backhitching, increases data retrieval time.
Furthermore, backhitching not only impairs performance, but also seriously impacts data reliability. In operation, backhitching induces extremely high transient forces that greatly increase tape wear and reduce the mechanical reliability of the drive.
Another limitation imposed by the track-following architecture is the significant area of the tape required to record track-following servo data. This servo overhead consumes physical space on the tape and limits the amount of data that can be written onto an individual tape.
Finally, the tight tolerances required for track-following technologies challenge the production margins of even the most sophisticated manufacturing facilities. Minute differences in assembly result in data availability and tape interchange issues. Nominal production variations, sometimes accentuated by differences in handling or environmental conditions, can make it impossible to exchange media between supposedly identical units from the same manufacturer. In other words, data written on one tape drive may not be read on another similar tape drive.
In view of the inherent limitations of track-following tape control architectures and need in the tape drive industry to keep pace with the disk drive industry in terms of performance, capacity, and cost, a need exists for a new methodology for delivering the required performance and capacity while reducing costs to the end-user.
The present invention is a novel method and apparatus for varying track recording speed to maximize host-to-tape data transfer rates. Variable data transfer rates of host systems and networks are accommodated by continually adjusting the tape speed to match the tape drive to the host""s actual transfer rate. This speed-matching capability eliminates backhitching along with its associated delays and reliability impacts.
In accordance with the invention, the speed of the tape is adjusted according to both the level of data present in the tape drive data buffer and whether the current mode of the drive is write mode or read mode. In the preferred embodiment, when the tape speed is accelerated or decelerated while writing a number of data track pairs to tape, a predetermined number of dummy track pairs are first written to tape at the current speed. On the next write head phase after the speed is increased or decreased as appropriate while the read heads are over the track, a predetermined number of dummy track pairs followed by more data track pairs are written to the tape at the new speed. Any resulting previously recorded track residue zones or overlap zones do not affect the reliability of any data written onto the tape since they occur between dummy track pairs, a detectable condition in read mode.
The present invention eliminates larger data errors that occur with traditional designs due to limitations of track following servo system and track curvature. These errors historically flow from tension variations, mechanical misalignment, interchange tolerances, and tape edge wear and damage. The tape speed control is based upon the maximum data transfer rate to the host computer or the fastest rate the device can receive data on reading. For writing operations, the speed control is based upon either the fastest rate the system will support or the speed data is received from a host computer. The speed of a remote host is matched for data transfer although data reading from the tape is possible even with the tape stopped.